Purification of individual chromosomes by flow sorting is often a first step in defining the structure of normal and abnormal human genes. DNA amplification techniques have greatly reduced the amount of sorted material required, making it feasible to develop smaller, lower-cost sorters. The performance of a dual-beam instrument using a single air- cooled helium-cadmium (He-Cd) laser as a source of both ultraviolet (UV) and blue excitation light for flow Cytometry of chromosomes stained with DAPI/olivomycin or Hoechst 33258/chromomycin A3 will be evaluated to determine whether the system can resolve the human karyotype as well as can existing commercial apparatus. In addition, several A-T and G-C preferring DNA fluorochromes not previously applied to chromosome analysis and potentially usable in a large number of existing flow sorters with single-beam 488 or 515 nm excitation will be examined as alternatives to stains presently used for bivariate flow karyotyping.